The present invention generally relates to fuel-fired heating apparatus and, in a representatively illustrated embodiment thereof, more particularly relates to a specially designed electronic water level sensing apparatus illustratively operative to sense the water level in a condensate collector box portion of a fuel-fired condensing type air heating furnace.
In a conventional gas-fired condensing type air heating furnace it is necessary to detect when the condensate (water) drainage from a collector box portion of the furnace is blocked. A draft inducer fan pulls discharged heating coil combustion products out of the interior of the collector box while it receives condensate from the coil. Operation of the draft inducer fan creates within the collector box a negative pressure which is conventionally detected by a normally open pressure sensor switch through a hose interconnected between the pressure switch and a pressure tap positioned at the lowest point on the collector box. The detected negative pressure maintains the switch in a closed position and is indicative of normal draft inducer operation and condensate drainage from the collector box.
If condensate drainage from the collector box becomes blocked, the condensate level in the box rises until it covers the pressure tap location, at which point the lowered pressure within the collector box is no longer transmitted to the pressure switch which responsively returns to its normally open state. This, in turn, opens the circuit of an associated condensate water level sensing system which responsively closes the furnace's gas valve and shuts down furnace operation.
For a multi-position furnace (i.e., one that can be positioned in an upflow, downflow, horizontal left or horizontal left orientation), the furnace is basically flipped or rotated in the field from one available orientation to another available orientation. For example, to change the furnace's orientation from upflow to downflow, the furnace is simply rotated 180 degrees from its upflow orientation to its downflow orientation in which the top side of the collector box becomes its bottom side.
Various problems, limitations and disadvantages, some of which are listed below, are inherent in this conventional pressure-based detection of the condensate level in a condensing furnace condensate collector box.
1. When the furnace is configured to, for example, upflow from downflow, the pressure used to determine if a blocked collector box drain condition exists must still be detected from a lowermost portion of the collector box which is now what was previously an uppermost portion of the collector box. This necessitates an undesirable downward field repositioning of the inlet end of the pressure switch hose to a lowermost section of the collector box.
2. Such representative repositioning of the furnace from an upflow orientation to a downflow orientation now places the pressure switch (which is normally placed at a level higher that that of the anticipated maximum level of condensate in the collector box to prevent condensate from flowing down the tube into the sensor) below the anticipated condensate level in the collector box. This necessitates another undesirable field reconfiguration—namely, the upward repositioning of the pressure sensor on the re-oriented furnace.
3. Another problem with this conventional pressure-based condensate level detection system is that it takes a long time to develop fully in the laboratory before it can be implemented in production. The reason for this is that determining the pressure switch setpoint for a given furnace is tedious and time consuming and depends on the furnace input BTU's, the maximum and minimum venting specified for the furnace, the installation altitude of the furnace, and many other factors.
4. Because of all of the various factors affecting the pressure switch setpoint, each furnace in a platform of furnaces (usually only distinguished by different input BTU's), normally must have a different pressure switch setting for the blocked drain sensing. This results in the necessity of stocking and inventorying additional unwanted parts at the factory, and for replacement parts, and leads to the possibility of an installer using the wrong setting for a given furnace.
5. For high altitude applications, the pressure switch used to sense the blocked collector box drain may need to be changed, or venting lengths must be reduced to compensate for the change in air density.
6. The pressure switch is a mechanical device with moving parts. It cannot be as reliable as typical solid state components without moving components. The contacts are subject to wear and corrosion which may prevent the switch mechanism from closing the circuit. Debris can also prevent the contacts of the switch from closing. Another problem is the possibility of welded or sticking contacts in the switch.
7. Because of the relative complexity of the pressure switch (a mechanical device), the cost can be high.
8. The pressure switch is a device which detects pressure used to assume that that excess water is not present in the collector box. Because it requires that a sensor used to detect pressure is used to detect the presence of water, the sensor is once removed from the actual thing that it is trying to detect, requiring assumptions which may not be valid (e.g., no pressure sensed means no water present). A more ideal device would directly sense the thing or condition (in this case, the presence of condensate) that needs to be sensed. Because the pressure switch is once removed from the parameter it is trying to detect, the measurement of the parameter requires undesirable additional tolerances and is subjected to similarly undesirable additional failure modes. One example is that of the necessary pressure switch tubing which may be blocked unexpectedly. The blocked tubing is not indicative to water drainage blockage, but still is detected as blocked water drainage because it unnecessarily prevents operation even though there is no blockage of condensate drainage from the collector box.
9. Because the pressure switch is not integral to the collector box, and because the pressure switch is relatively large, it must usually be mounted remotely from the utilized sensing port on the collector box and connected to the port with a hose. Due to limited space availability within the furnace, it is often difficult to find enough free area/space to mount the pressure switch. A more ideal sensor would be small enough to mount directly to the port.
10. Pressure switches used for this application must have what is known as a bleed port. The bleed port is a small hole in the pressure switch which allows a tiny amount of air to pass through the pressure switch and its associated hose or tube. Without a bleed port, no air would pass through the hose. As the hose and pressure switch are exposed to the wet environment of the furnace, they would eventually become filled with water unless some air was allowed to pass through to keep them dry. This is the function of the bleed port. However, because of this bleed hole, the small amount of air allowed to pass through can allow enough negative pressure to keep the pressure switch closed even when the port on the collector box is covered with water. Once the water level in the collector box gets high enough over the port, the water pressure is too great for this small amount of air to overcome. This means that the water level must get significantly above the port on the collector box before the blocked condition is sensed. If the water level is high enough, it can get into the secondary tubes of the condensing heat exchanger, which is an undesirable condition. Because of this, the distance between the port on the collector box and the lowest tube on the heat exchanger needs to be greater than would be necessary if the water level could be detected at the port level. Since space in a furnace is a problem, this added distance requires that the furnace must be larger than necessary.
As can be readily seen from the foregoing, it would be desirable to provide a fuel-fired condensing type air heating furnace with an improved condensate water level detection system. It is to this goal that the present invention is primarily directed.